I don't know why---but I do know many people have had the standard TV antenna switched out. For one my brother could not pick up any channels with the one that came on his unit. Had it switched and now seems to be fine. Now mine on the other hand worked fine for a few months after I took delivery, but then seemed to fade out and quit working. I had a replacement sent to me frome CR but have not had a chance to see if it works yet. If you do a search for "Antenna" on this board you will find plenty of people complaning about the Omni-directional antenna.
F250-7.3 L, 2012 305SK--Happy Wife, Good Life